纤维素热解过程中糖苷C–O键的活化：α与β和缩合相羟基催化裂解,ACS Catalysis

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纤维素热解过程中糖苷C–O键的活化：α与β和缩合相羟基催化裂解
ACS Catalysis ( IF 11.3 ) Pub Date : 2020-07-06 , DOI: 10.1021/acscatal.0c02133
Vineet Maliekkal ₁ , Paul J. Dauenhauer ₁ , Matthew Neurock ₁

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通过第一原理密度泛函理论计算获得了对纤维素热解中糖苷键活化的机理的见解，这些原理解释了不同立体化学糖苷键（βvsα）动力学上的独特相似性，并确立了反应周围三维羟基环境的作用处于激活动态的中心。据报道，据报道，α-异构体的活化机制要求初始形成瞬态C ₁ -O ₂ -C ₂环氧化物，随后将其转变为左葡糖聚糖。麦芽糖（α-异构体的模型化合物）的密度泛函理论结果表明，分子内C ₂羟基以类似于羟甲基（C ₆羟基）与β糖苷键的醚氧原子上的孤对电子相互作用的方式有利地与α-糖苷键的醚氧原子上的孤对电子相互作用。该机理的活化能为219 kJ mol ^–1，类似于非催化转糖基化机理报道的障碍（〜209 kJ mol ^–1）。）并与实验测得的高温下α-环糊精转化的势垒高度吻合。结果有助于解释解聚动力学缺乏对糖苷键立体化学的敏感性。随后的受限制的从头算分子动力学（AIMD）模拟显示，反应的碳水化合物的冷凝环境中的邻位羟基通过2至3个氢键熔化了锚定过渡态，并导致较低的自由能垒（〜134–155 kJ mol ^{– 1}）与先前的实验一致。

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Glycosidic C–O Bond Activation in Cellulose Pyrolysis: Alpha Versus Beta and Condensed Phase Hydroxyl-Catalytic Scission

Mechanistic insights into glycosidic bond activation in cellulose pyrolysis were obtained via first-principles density functional theory calculations that explain the peculiar similarity in kinetics for different stereochemical glycosidic bonds (β vs α) and establish the role of the three-dimensional hydroxyl environment around the reaction center in activation dynamics. The reported activating mechanism of the α-isomer was shown to require the initial formation of a transient C₁–O₂–C₂ epoxide that subsequently undergoes transformation to levoglucosan. Density functional theory results from maltose, a model compound for the α-isomer, show that the intramolecular C₂ hydroxyl group favorably interacts with lone pair electrons on the ether oxygen atom of an α-glycosidic bond in a manner similar to the hydroxymethyl (C₆ hydroxyl) group interacting with the lone pair electrons on the ether oxygen atom of a β glycosidic bond. This mechanism has an activation energy of 219 kJ mol^–1, which is similar to the barriers reported for noncatalytic transglycosylation mechanism (∼209 kJ mol^–1) and in good agreement with experimentally measured barriers for α-cyclodextrin conversion at high temperatures. The results help explain the lack of sensitivity of depolymerization kinetics to glycosidic bond stereochemistry. Subsequent constrained ab initio molecular dynamics (AIMD) simulations revealed that vicinal hydroxyl groups in the condensed environment of a reacting carbohydrate melt anchor transition states via two-to-three hydrogen bonds and lead to lower free energy barriers (∼134–155 kJ mol^–1) in agreement with previous experiments.

更新日期：2020-08-08

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